<!DOCTYPE html>
<html>
    <head>
        <style>
            body,html{
                margin: 0;
                width: 100%;
                height: 100%;
            }
            .left{
                float: left;
                height: 100%;
                width: 500px;
                background-color: #cccccc;
            }
            .right{
                height: 100%;
                background-color: red;
            }
            
        </style>
    </head>
    <body>
        <div class="left">left</div>
        <div class="right">right</div>

    </body>
</html>